Dynamic behaviour of the keyhole in laser welding

Many papers have sought correlations between the parameters of secondary particles generated above the beam/work piece interaction zone, dynamics of processes in the keyhole, and technological processes. Low-and high-frequency oscillations of the current, collected by plasma have been observed above the welding zone during electron beam welding. Low-frequency oscillations of secondary signals are related to capillary instabilities of the keyhole, however; the physical mechanisms responsible for the high-frequency oscillations (>10 kHz) of the collected current are not fully understood. This paper shows that peak frequencies in the spectra of the collected high-frequency signal are dependent on the reciprocal distance between the welding zone and collector electrode. From the relationship between current harmonics frequency and distance of the collector/welding zone, it can be estimated that the draft velocity of electrons or phase velocity of excited waves is about 1600 m/s. The dispersion relation with the properties of ion-acoustic waves is related to electron temperature 10 000 K, ion temperature 2 400 K and plasma density 10 16 m −3 , which is analogues to the parameters of potential-relaxation instabilities, observed in similar conditions. The estimated critical density of the transported current for creating the anomalous resistance state of plasma is of the order of 3 A · m −2 , i.e. 8 mA for a 3-10 cm 2 collector electrode. Thus, it is assumed that the observed high-frequency oscillations of the current collected by the positive collector electrode are caused by relaxation processes in the plasma plume above the welding zone, and not a direct demonstration of oscillations in the keyhole. C 2014 Author(s)

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“…[22][23][24][25][26] These auto-oscillations are of stochastic nature and are determined by the internal properties of the system beam/keyhole.…”

Section: Introductionmentioning

confidence: 99%

Current-driven ion-acoustic and potential-relaxation instabilities excited in plasma plume during electron beam welding

et al. 2014

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“…Usually, the natural oscillation frequency of a partial penetration pool is higher than the natural oscillation frequency of a full penetration pool and an abrupt transition occurs between both modes of oscillation. 17,18) The free oscillation frequencies for a system consisting of a cylindrical keyhole surrounded by a concentric cylinder of liquid should occur in the 0.5-1 kHz range, 19,20) and is the highest among the three phases. As a result, the area below the curve of the low frequency band (less than 200 Hz) of the power spectral density is different on the three phases, which is the largest on the transition phase.…”

Section: Discussionmentioning

confidence: 99%

Plasma-arc Welding Sound Signature for On-line Quality Control.

Wang

Zhao

2001

ISIJ International

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On-line quality control in automated welding operations is an important factor contributing to higher productivity, lower costs and greater reliability of the welded components. The sound signal of plasma arc welding was acquired at high speed and investigated with the aid of computers. It is shown that the amplitude of the sound signal greatly varies with the variation of the status of the weld pool. The analysis of frequency domain indicated that the area below the curve of the low frequency band of the power density spectra, which may reflect the oscillation properties of the weld pool, is largest on the transition phase. The high frequency dominant peaks of the sound signal, which may relate to the plasma jet pulsation, is highest on the keyhole-formed phase. The method for detecting the behaviors of the weld pool keyhole was developed. The experimental results indicate that the acoustic emission is a usable, practical information source in the penetration quality detection of plasma arc welding.KEY WORDS: plasma arc welding; power spectrum; sound signature; process control. Fig. 1. Schematic drawing of the experimental system.

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“…The influence of zinc vapour evolution on keyhole elongation may be explained from an analysis of the pressure balance at the keyhole wall. For a stationary keyhole with a high aspect ratio, the following pressure balance is given by kroos et al [32,33]:…”

Section: Analytical Model Of Keyhole Elongationmentioning

confidence: 99%

Keyhole behaviour during laser welding of zinc-coated steel

Pan¹,

Richardson²

2011

J. Phys. D: Appl. Phys.

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The production of consistent, high-quality laser welds on zinc-coated steels for the automotive industry remains a challenge. A simple overlap joint geometry is desirable in these applications but has been shown to be extremely detrimental to laser welding because the zinc vapour formed at the interface between the two sheets expands into the keyhole and disrupts fluid flow in the melt pool, which often leads to metal ejection. In this work, laser welding on sheets with various coating thicknesses has been performed and it is observed that the sheets with thick coatings (∼20 µm) show surprisingly good weldability. High speed video camera visualizations of the keyhole provide insight into the keyhole dynamics during the process. It appears that the dynamic pressure of zinc vapour can effectively elongate the keyhole and the process can reach a stable state when an elongated keyhole is continuously present. A simple analytical model has been developed to describe the influence of zinc vapour on keyhole elongation.

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Dynamic behaviour of the keyhole in laser welding

Cited by 104 publications

References 6 publications

Current-driven ion-acoustic and potential-relaxation instabilities excited in plasma plume during electron beam welding

Current-driven ion-acoustic and potential-relaxation instabilities excited in plasma plume during electron beam welding

Plasma-arc Welding Sound Signature for On-line Quality Control.

Keyhole behaviour during laser welding of zinc-coated steel

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